gerund - What does "a-building" modify?

I would dare to say that "a-building" is no adjective and "a-hunting no participle but an old gerund form with the rest of a preposition as prefix.

a) We went a+hunting. Compare German: Wir gingen zum Jagen (word-for-word translation: We went to the hunting.) So we may assume that in English "to hunting" was reduced to "a-hunting".
These old forms of a- + gerund can be found in older literature.

Occasionally you find expressions such as "A storm was brewing". In this expression "brewing" has no active meaning but rather a reflexive meaning. One might give the idea by saying:
"A storm was brewing itself". And such expressions were at first used with a preposition such as "in/on/at". Let us assume it was: A storm was in (the act of) brewing itself.

This is meant to give an understanding of such curious expressions as in the above sentence:
"The city was a thousand years a-building."